Gas turbine engines operate to produce mechanical work or thrust. For land-based gas turbine engines, a generator is typically coupled to the shaft, such that the mechanical work produced is harnessed to generate electricity. A typical gas turbine engine comprises a compressor, at least one combustor, and a turbine, with the compressor and turbine coupled together through an axial shaft. In operation, air passes through the compressor, where the pressure of the air increases and then passes to a combustion section, where fuel is mixed with the compressed air in one or more combustion chambers. The hot combustion gases then pass into the turbine and drive the turbine. As the turbine rotates, the compressor turns, since they are coupled together along a common shaft. The turning of the shaft also drives the generator for electrical applications. The gas turbine engine also must operate within the confines of the environmental regulations for the area in which the engine is located. As a result, more advanced combustion systems have been developed to more efficiently mix fuel and air so as to provide more complete combustion, which results in lower emissions.
Low emissions combustion systems require the fuel and air being mixed to be properly proportioned in order to obtain optimal results. Fuel flows are usually tightly controlled through carefully sized orifices in the fuel nozzles and controlled fuel valves. Airflows may actually vary due to distributions driven by the compressor exit profile and the amount of air required to cool the turbine section. Because the amount of air introduced into the combustion system significantly affects reaction zone temperature and performance of the combustion system, an adjustable air mass is advantageous for regulating the combustion process.
A general issue with gas turbines, and especially industrial gas turbines, is the need to be able to tune the combustors to avoid issues such as lean blow out (LBO), where the combustor is operating too lean and is not receiving enough fuel, for a given amount of air, causing the flame to be extinguished. Another known problem of tuning a gas turbine combustor include excessive combustion dynamics caused by rapid changes in pressures within the combustor.
To compensate and control these combustion instabilities, prior gas turbine combustors incorporated additional dilution holes in the combustion liner or a transition piece in order to control the amount of air being used in the combustion process. However, these forms of “air control” have been known to adversely effect emissions of the combustion system, at least with respect to carbon monoxide.